MPS or Oxidizer (also called spa shock) is a non-chlorine shock treatment that eliminates odors and reduces irritating contaminants, to leave water sparkling clear. When used with the FreshWater Ag+ purifier, this product enhances the activation of the ions to work faster. It won't alter the chemistry of the water. While it works similarly to Chlorine, Shock does not replace chlorine. For best results, use Chlorine on a weekly basis and Shock 10 minutes prior to using your spa.